Method for providing extra-traffic paths with connection protection in a communication network, related network and computer program product therefor

ABSTRACT

Extra traffic paths are provided in a communication network including at least two protection channels associated to respective transmission channels. Each protection channel admits an active state for carrying, in the presence of a failure in the associated transmission channel, traffic to be carried by the associated transmission channel, and a stand-by state, wherein the protection channel is adapted to carry extra traffic. The protection channels are run in a sub-network connection protection scheme, whereby one of the protection channels in its stand-by state is adapted to ensure recovery of extra traffic carried by the other protection channel while the other protection channel is switched to its active state or is subject to failure.

FIELD OF THE INVENTION

The invention relates to telecommunication networks and was developed bypaying specific attention to the possible application to ringarchitectures in SDH (Synchronous Digital Hierarchy) networks of thetype currently designated MS-SPRing.

Reference to this possible field of application is not however to beconstrued as limiting the scope of the invention.

DESCRIPTION OF THE RELATED ART

Long distance transmission networks provide a wide variety of differenttelecommunication services, such as voice and Internet data or leasedlines at different speeds.

These networks generally include a plurality of nodes, each typicallylocated in a city or other high traffic location, coupled together in aclosed loop or a ring/mesh architecture e.g. by fiber optic cables. Theinformation travels over the fiber according to optical transmissionstandards such as e.g. those currently referred to as SynchronousDigital Hierarchy (SDH) or Synchronous Optical Network (SONET).

In ring architectures, traffic protection can be easily performedcreating two separate links over the loop and dedicating the former tonormal flow and the latter to protection, in order to ensure servicewhen a fault event occurs in the former link.

This protection arrangement is currently designated sub-networkconnection protection (SNCP). In this kind of protection arrangement nospecific mechanisms exist for coordinating both ends of the link. Thismeans that no specific protocol exists for exchanging informationbetween protection terminations about “switch” and “bridge” (e.g.protection) states.

One possible implementation of a sub network connection protectionscheme involves two equipments that carry traffic along a certaininfrastructure (e.g. a ring). The network element at the source end ofthe protection span transmits the protected channel over the two sidesof the ring as a protection group. The network element at the sink endof the protection span outputs the received protection group as a choiceof the “best” sides depending by switching criteria. In case no defectsare detected in the working SNC/trail signals, these signals areselected as the normal signals. The SNCs/trails selected by the sink endare called the active SNCs/trails, while the others are called thestandby SNCs/trails.

Trail protection schemes exist (such as the scheme designated MultiplexSection Protection or MSP) that offer the possibility of using an inband protocol in order to synchronize both terminations. This featuremay be exploited to establish channels adapted to be routed via theprotection trail/section/path in stand-by condition. The signalsconveyed over such channels are usually called extra traffic (ET), seee.g. ETSI EN 300 417-1-1, v1.2.1, (2001/10), “Transmission multiplexing;generic requirements of transport functionality of equipment; Part1-1:generic process and performance; page.18.

A so-called MS-SPRing protection scheme can be implemented by means of atwo or four fiber Multiplex Section Shared Protection Ring (MS-SPRing).On each span, half of the capacity (with one or two couples of fibers)is dedicated to “working” channels and the other half to “protection”channels. Traffic can flow over the ring in a clockwise orcounter-clockwise direction. In case of a single fiber failure, a spanprotection scheme is applied, and traffic is re-routed along theprotection span adjacent to the failed one. In case of second orderfaults (e.g. working and protection fiber failures or node failure) aring protection scheme is implemented and traffic is re-routed along thenon-failed side of the ring.

FIG. 1, including four portions designated a) to d) respectively,schematically represents the operation of a typical MS-SPRing protectionscheme involving four nodes designated a, n1, n2, and Z.

In FIG. 1, “working” and “protection” fibers are shown, the protectionfibers being represented by hashed areas and failure events by crosses.

Specifically, in FIG. 1 the four following conditions are shown:

(a) normal operation;

(b) span protection active;

(c) ring protection against double span failure and;

(d) ring protection against node isolation

In addition to the normal MS-SPRing protection algorithm (which offersprotected channels with high priority levels), the possibility exists ofexploiting non-preemptible unprotected traffic (NUT) channels to offeran intermediate priority level and extra traffic (ET) channels havinglow priority levels.

In particular, some working and protection channels in a MS-SPRingarrangement can be used to carry NUT, which is unprotected traffictraveling on specific channels on working and protection paths of thering. In case of failure, these channels cannot be cut off to supportprotection of normal traffic.

In bi-directional protection schemes, where a termination coordinationprotocol is available, the band normally reserved to protection can beexploited to carry extra traffic when the protection channel is instandby mode. Obviously, when a failure event occurs on the workingpath, and the protection scheme switches from the working to theprotection path, the extra traffic channel is pre-empted and itsrecovery is not guaranteed.

As shown in FIG. 2, a 1+1 protection architecture has one normal trafficsignal, one working SNC/trail, one protection SNC/trail and a permanentbridge.

A m:n protection architecture has n normal traffic signals, n workingSNCs/trails and m protection SNCs/trails. The signals on the workingSNCs/trails are the normal traffic signals. The signal on a protectionSNC/trail is either one of the normal traffic signals, an extra trafficsignal, or a null signal. At the source end, any of the signalsmentioned can be connected to the protection SNCs/trails. At the sinkend, the signals from the working SNCs/trails are selected as the normaltraffic signals. In case of a defect condition on a working SNC/trail,the transported signal is routed over one of the protection SNCs/trails.At the sink end, the signal from this protection SNC/trail is thenselected instead.

The recommendation ITU-T G.841 “Series G: Transmission systems andmedia, digital systems and network—Types and characteristics of SDHnetwork protection architectures”, October 1998, indicates that afurther differentiation on priority levels may be achieved byconsidering a sub network connection protection mechanism partiallyembedded in a MS-SPRing using NUT. In other words, NUT may be protectedalong the MS-SPRing with the implementation of a SNCP path. This kind ofprotection preserves NUT from double failures over a span.

NUT channels are however affected by two main disadvantages, namely:

for using a NUT channel, it is necessary to create more paths thanneeded; for example, for a NUT channel on the working bandwidth, it isnecessary to dedicate it the slot number one of all the working andprotection spans forming the ring;

for creating a non-planned link in a NUT configuration, it is necessaryto perform a ring re-commissioning.

In WO-A-02/073903 an arrangement is disclosed for achieving anavailability differentiation through the definition of a differenttechnology also known as resilient packet ring (RPR) technology. In thisarrangement, different availability levels are obtained by definingthree classes of service each one having a different priority level. Thethree classes of traffic are the following: protected traffic (i.e. highpriority); unprotected traffic (i.e. medium priority); and emptabletraffic (i.e. low priority).

The main disadvantage of this solution is that operators wishing todevelop network facilities to deploy different levels of service will beinevitably forced to change equipment with a great impact on the capitalexpenses.

Document WO-A-01/030006 describes a variant of the SNCP protectionscheme discussed in the foregoing providing another implementation of a1+1 protection scheme. Specifically, the possibility of carryingun-protected and un-rerouted extra traffic signals on the protectionpath of a 1+1 sub network connection protection scheme is suggested,when the protection trail is in a stand-by condition.

OBJECT AND SUMMARY OF THE INVENTION

The need therefore exists for improved arrangements that may be adaptedto create paths with different availability levels by dispensing withthe disadvantages intrinsic in the prior art arrangement discussed inthe foregoing.

The object of the present invention is thus to provide such an improvedarrangement.

According to the invention, such an object is achieved by means ofmethod having the features set forth in the claims that follow. Theinvention relates to corresponding system, as well as a correspondingcomputer program product loadable in the memory of at least one computerand comprising software code portions for performing the steps of themethod of invention when the product is run on at least one computer.Reference to “at least one” computer is intended to highlight thepossibility for the arrangement of the invention to be carried out in ade-centralized manner.

A preferred embodiment of the invention is based on an improved way toprotect extra traffic (ET) channels, by exploiting a sub networkconnection protection scheme applied to the MS-SPRing architecture.

Protection is based on the principles underlying the functional model ofITU-T Recommendation G.803.

As used herein, the intended meaning of “sub network connectionprotection” is not limited to the specific arrangement currentlyidentified by the acronym SNCP, but also extends to equivalentprotection schemes such as e.g. those known as MS-SPRing, Restorationand Multi Protocol Label Switching (MPLS) or Generalized MPLS.

Such a protection scheme preferably makes use of pre-assigned capacitybetween nodes. The simplest architecture has 1 working and 1 protectioncapacity (1+1), the most complex architecture has n working capacitiesand m protection capacities (m:n), respectively.

The arrangement described herein offers the possibility of creating apath protected service using two low priority extra traffic (ET)channels present in a four-fiber MS-SPRing arrangement. Extra trafficchannels may be protected in a MS-SPRing structure by doubling an extratraffic channel along the clockwise and counter-clockwise sides of thering and applying a sub network connection protection protocol.

This strategy of protection offers an opportunity for furtherdifferentiating the service priority levels. In fact, sub networkconnection protection on extra traffic channels allows recovery of sucha low priority traffic in case of a single failure along any span of thering. As a consequence, a new intermediate level of protection isachieved among the levels assured by NUT and SNCP protected NUT.

As indicated, NUT channels have two main disadvantages, namely:

for using a NUT channel, it is necessary to reserve more bandwidth thanneeded; for example, for a NUT channel on the working bandwidth, theslot “number one” slot of all the working and protection spans formingthe ring must be dedicated to it; and

in order to create a non-planned link in NUT configuration, it isnecessary to perform a ring re-commissioning.

Referring to the disadvantages of NUT channels described in the previoussection, extra traffic does not permanently reserve a channel along theprotection path.

The arrangement described herein is technology independent. This meansthat only two basic requirements for the network protocol oftransmission exist, namely:

having a form of circuit protection; and

having the possibility of configuring low priority traffic with respectto the normally protected one.

The matching between low priority path and sub network connectionprotection capabilities can be achieved either on the same networkequipment or on a different equipment. In other words, almost all thenetwork equipments having low priority traffic capabilities are adaptedto use the combination with SNCP protection.

By way of direct comparison, while the arrangement shown inWO-A-01/030006 discloses the possibility of carrying extra traffic overa 1+1 SNCP protection scheme (which i.a. is not described in normativedocuments), in the arrangement described herein the protection path of aMS-SPRing scheme is exploited to carry extra traffic signals and amethod to protect ET by a 1+1 SNCP implementation scheme is shown.

A preferred embodiment of the arrangement disclosed herein thus providesextra traffic paths in a communication network including at least twoprotection channels associated to respective transmission channels.

Each protection channel admits:

an active state for carrying, in the presence of a failure in saidassociated transmission channel, traffic to be carried by the associatedtransmission channel, and

a stand-by state, wherein the protection channel is adapted to carryextra traffic.

The protection channels are run in a sub-network connection protectionscheme, whereby one of the protection channels in its stand-by state isadapted to ensure recovery of extra traffic carried by the otherprotection channel while the other protection channel is switched to itsactive state or is subject to failure.

Preferred embodiments of the arrangement disclosed herein provide:

at least one ring structure including non-coextensive paths andassociating the at least two protection channels to respectivenon-coextensive paths in the ring (i.e. by resorting to what iscurrently designated “routing diversity”, and

a plurality of ring structures and associating the at least twoprotection channels to respective, different rings of the plurality ofrings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, byreferring to the enclosed figures of drawing, wherein:

FIGS. 1 and 2, related to the prior art, has been described previously,

FIG. 3 shows an SNCP protected path through two extra traffic channelsbetween two different nodes,

FIG. 4 shows a ring arrangement implementing a four fiber MS-SPRingprotection scheme,

FIGS. 5 and 6 show two possible failure events occurring in thearrangement of FIG. 4,

FIG. 7 shows two extra traffic channels protected with a sub networkconnection protection scheme,

FIG. 8 shows an alternative arrangement to the arrangement of FIG. 7,

FIGS. 9 and 10 show various arrangements for routing differentiationover two rings,

FIGS. 11, 12 and 13 show various configurations foravailability/unavailability evaluation,

FIG. 14 is a general representation of extra traffic protected via a subnetwork connection protection scheme.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

FIG. 3 of the drawing is a representation of a SNCP protected pathbetween two equipments 10 and 12.

A backbone network based on a ring interconnected architecture with fourfibers MS-SPRing protection can offer a wide network service portfoliobased on different levels of availability. The arrangement describedherein enlarges the possibilities of service level differentiation on aMS-SPRing SDH network.

This arrangement is essentially the application of the SNCP protectionscheme to extra traffic channels A, B in a network N extending betweentwo equipments 10 and 12. The two extra traffic channels A and B aresent along the working and protection trails of a SNCP protectionarchitecture. Specifically, FIG. 4 shows a SNCP protected path includingtwo extra traffic channels between two different nodes 10 and 12.

FIG. 4 shows a four fiber MS-SPRing protection scheme architecture, withextra traffic on the bandwidth of the protection sections. Specifically,FIG. 4 (and FIGS. 5 to 8 as well) refer to a ring arrangement providedbetween two nodes 100, 200 including respective data exchange control(DXC) modules and including four add-drop multiplexers ADM1 to ADM4. Twoextra traffic channels are depicted, routed on the two opposite sides ofthe ring.

In the ring with four fiber MS-SPRing protection scheme, the bandwidthof the protection paths may be dedicated to extra traffic as shown withtwo extra traffic channels, on the two opposite sides of the ring. Thepaths terminate on two digital cross connects.

FIG. 5 shows interruption of channel number 1 when a span failureoccurs, with channel number 2 working normally and the state of the twoextra traffic channels in case of span protection: path number 1 is outbecause the protection path is devoted to normal traffic spanprotection, while path number 2 is regularly in service.

FIG. 6 shows interruption of channels number 1 and number 2 when adouble span failure occurs. The ring works in ring protection mode inwhich both extra traffic paths have been empted to allow the re-routingof high priority traffic.

Service level agreement currently require that, when this faultycondition occurs, ring fibers and/or equipments must be repaired in afew hours (typically less then one work day).

As already mentioned in the foregoing, referring to NUT channels, afurther differentiation on priority levels may be achieved by acomposite solution, using NUT channels in a four fiber MS-SPRing with anoverlaid sub network connection protection scheme. In other words, NUTmay be protected along the MS-SPRing with the implementation of a subnetwork connection protection path. This kind of protection preservesNUT from double failures along a span.

The different solutions analyzed so far are not exempt from certaincritical aspects.

NUT channels allow both protected and unprotected channels on the sameinfrastructure but, when a ring is in the commissioning phase,necessitate exactly planning the bandwidth requirements because anydifference implies a new re-commissioning phase in order to optimize thereused ring bandwidth.

Sub network connection protection of NUT channels implies that, in thenetwork planning phase, i.e. in the ring commissioning phase, the NUTchannels required to carry out traffic demands must be defined. For justone traffic demand it is necessary to define one slot for the whole ringas NUT (this implies that the bandwidth allocated is greater than theone effectively used)

Using extra traffic channels on a MS-SPRing infrastructure offers anavailability level that is much lower in comparison with the protectedchannels. In addition, the extra traffic availability is also affectedby long outage periods due to planned maintenance activity.

A preferred embodiment of the arrangement described herein offers thepossibility of creating a path protected service using two low priorityextra traffic channels present on four-fiber MS-SPRing.

As previously discussed in the case of NUT, extra traffic channels maybe protected in a MS-SPRing structure by doubling an extra trafficchannel along the clockwise and counter-clockwise sides of the ring andapplying a sub network connection protection protocol.

This protection scheme offers an opportunity to further differentiatethe service priority levels. In fact, sub network connection protectionof extra traffic channels allows the recovery of low priority traffic incase of a single failure along any span of the ring.

Availability evaluation shows that the protection level obtained ishigher than in the case of simple NUT channels (this is because the pathis protected on the two different routes of the ring) but lower withrespect to SNCP protected NUT.

In comparison with the NUT implementation, an advantage of using extratraffic channels with an sub network connection protection scheme liesin planning the ring capacity for extra traffic services, so acommissioning activity is not required any time an extra traffic channelis to be provisioned.

Within this context, at least two embodiments are possible, namely:

two extra traffic channels protected with a sub network connectionprotection scheme having the bridge/selector function performed on thesame add drop multiplexers (ADMs) forming the ring; and

two extra traffic channels protected with an SNCP scheme having thebridge/selector function performed on digital cross connects (i.e.DXCs).

Specifically, FIG. 7 shows two extra traffic channels protected with asub network connection protection scheme having the bridge/selectorperformed on the same add drop multiplexers (ADMs) forming the ring.

Conversely, FIG. 8 shows two extra traffic channels protected with a subnetwork connection protection scheme having the bridge/selectorperformed on digital cross connects (DXCs).

In the implementation of the sub network connection protection schemeover the add drop multiplexers (ADMs) forming the ring infrastructurethat carries traffic patterns, a sub-network termination point iscreated that includes two extra traffic paths routing over the twoopposite sides of the ring, as represented in FIG. 4.

In the implementation of the sub network connection extra trafficprotection scheme over the DXCs, the equipment represents the pathtermination, in a specific network domain, of a path created by twoextra traffic sub-paths protected via a sub network connectionprotection scheme. In this latter case, different possibilities existfor establishing the extra traffic services, that depend on the routingstrategy applied to the infrastructure (e.g. routing via minimum numberof rings.

Specifically the following situations may arise:

in case of extra traffic paths protected via the SNCP algorithm over asingle ring, as described above, the SNCP mechanism can be configuredover the ADMs while the ring infrastructure deploys the ET paths inrouting diversity (see again FIG. 7);

in case of extra traffic paths protected via the sub network connectionprotection algorithm over the same ring, the sub network connectionprotection mechanism can be configured over the DXCs as a variant of theprevious scheme (see, in that respect, FIG. 8);

in case of extra traffic paths protected via SNCP algorithm overdifferent rings, the sub network connection protection mechanism can beconfigured over the DXC and the ring infrastructure should berepresented as extra traffic paths carried over two different ringsbelonging to the same class (i.e. a ring number “one” an a ring number“two”)

Extra traffic paths can also be carried over two rings belonging todifferent ring classes (i.e. a ring number “one” of class A and a ringnumber “one” of class B).

In respect of this last mentioned option, it is important to understandthat this strategy achieves an improvement in terms of pathsurvivability and, specifically, offers good protection level when aplanned maintenance activity affects the operability condition of one ofthe extra traffic paths used in SNCP protection scheme.

In order to reach the maximum theoretical advantage from extra trafficpaths carried out over two different rings of the same class, theworking and the protection paths are planned on different routing. Ifthe different routing is represented by two rings that belong to thesame class (over the same cable infrastructure are deployed two or moredifferent ring systems, a. A way of obtaining routing diversification isto route one extra traffic path on one side of the first ring and thesecond extra traffic on the opposite side of the second ring. This meansthat only two faults over the same span (working and protection) maylead to an out of service condition.

Specifically, FIG. 9 shows routing differentiation over two rings of thesame class, which leads to a significant advantage from the extratraffic protection via a sub network connection protection scheme.

In the case of extra traffic paths carried out over two rings belongingto different ring classes, a complete diversification of routing (i.e.different equipments and infrastructures) is possible, so that theavailability level reaches its maximum.

In that respect, FIG. 10 shows routing differentiation over two rings ofthe same class, which leads to a maximum advantage from the extratraffic protection via a sub network connection protection scheme.

It will be appreciated that two additional ADMs (namely ADM5 and ADM6)are shown in ring number one of class Y in order to highlight that thering structures considered herein may include any number of networkequipments as provided for in the ITU-T G.841 specification.

The following is a discussion of investigations performed on a ringstructure interconnecting two geographical sites (again designated 100and 200) with two different paths named red (R) and blue (B) directions.

The unavailability figures of NUT and extra traffic (ET) channels havebeen calculated in three cases.

In the first case (FIG. 11), the calculation is performed on apoint-to-point link, along the red (R) or blue (B) paths.

In the second case (FIG. 12), two NUT or ET channels have been createdalong the red (R) and blue (B) paths, in order to perform a sub networkconnection protection scheme (the shortest path has been considered as“working” and the longest one as “protection”).

In the third case (FIG. 13) the sub network connection protection schemeis performed on the DXC equipment, thus allowing the infrastructurediversification of the working and protection channels (ring 1 and ring2).

The equipments and infrastructure unavailability values are reported inTable 1 below TABLE 1 Unavailability Network element Functional lock[abs] ADM STM1 ports 1.36711E−05 Matrix 1.37079E−10 Common parts3.52434E−05 Line ports 2.53672E−05 RED STM1 ports 4.11369E−06 Matrix 1.0824E−09 Common parts 1.46034E−05 Ring characteristics Fibre/km0.000003 Rings circumference 1000 km n. ADM/ring 4

Table 2 summarizes the results obtained in the three cases describedabove. TABLE 2 Unavailability Availability Unav. Case Node A Node Z PathProtection type [abs] [%] min/year 1 A1 Z1 red ET 0.008218322 99.1804319 A2 Z2 blue ET 0.00900645 99.100 4733 A1 Z1 red NUT 0.0047427599.526 2493 A2 Z2 blue NUT 0.00545401 99.455 2867 2 A Z ring 1 ET SNCPADM 0.00011064 99.990 58 A Z ring 1 NUT SNCP 0.00002587 99.997 14 ADM AZ ring 1 MS-SPRing 0.00002055 99.998 11 protected ADM 3 A Z ring 1-2 ETSNCP RED 0.00013148 99.989 69 A Z ring 1-2 NUT SNCP 0.00004420 99.996 23RED A Z ring 1 MS-SPRing 0.00004175 99.996 22 protected RED A Z ring 2MS-SPRing 0.00004162 99.996 22 protected RED

The data show NUT channels to have fewer minutes of unavailability whencompared to the ET channels. For both types of protection, the order ofmagnitude ranges from the thousands of minutes in the non-protected caseto tens of minutes for the SNCP ones. The unavailability of ET channelsprotected SNCP stands in between the unavailability figures ofunprotected and SNCP protected NUT.

The arrangement described herein is adapted for use in meshedarchitecture networks where ET sub-paths use the bandwidth defined fortraffic restoration. This is shown, in quite general terms, in FIG. 14,where one such network is indicated by the reference number 1000.

The meshed infrastructure shown therein is implemented via digital crossconnects interconnected with line terminals. Each branch in the networkplan has associated a certain amount of bandwidth to recover equipmentor cable failures. When the network efficiency is failure exempt, theband reserved to recovery is unused and can be configured to carry outET. In order to increase the availability ratio it is possible toprotect two channels by an sub network connection protection scheme.

The focus of the arrangement described herein is on the protection ofpaths realized in low priority configuration. The embodiment describedconcerns SDH technology with ring or meshed architecture.

In this kind of network, data are transmitted via TDM (Time DivisionMultiplexing) in those protected channels (i.e. high priority channels)whose length is fixed (VC). Each channel carries one or more circuitdata flows defined by the implemented protocol.

Networks of this kind include, for example, SDH (Synchronous DigitalHierarchy) or SONET (Synchronous Optical Network) networks.

In this kind of network, with high priority channel, extra trafficchannels (i.e. low priority channels) can be configured in various ways.

In WDM (Wavelength Division Multiplexing) networks, data are transmittedon different wavelengths, which replace the multiplexing time slots.

With this technology it is possible to create the so called “lambda”protected paths; the lambda (i.e. the wavelength) reserved to theprotection of the main lambda, or in sharing with a bundle of otherlambdas, can be assigned the role of carrying low priority traffic. Froma technical point of view, extra traffic in SDH and SONET networks andlow priority lambdas in WDM networks can be considered equivalent. Theyare both emptable channels when a failure state occurs, regardless ofthe actual physical implementation of the network.

Also, SDM (Space Division Multiplexing) or FDM (Frequency DivisionMultiplexing) networks can be similarly implemented. In these cases theconcept of Extra Traffic paths is associated to intervals of space orfrequency. As is well known, FDM was the dominant technology before TDMtechnology was developed.

The invention refers to all types of networks based on the concept ofcircuit protection described above. Specifically, reference made in thedescription to TDM (SDH and SONET) and WDM networks is primarilydictated by their current popularity.

A restoration application makes use of any capacity available betweennodes. In general, restoration will involve re-routing. When restorationis used, a percentage of the transport network capacity will be reservedfor re-routing working traffic. Restoration is initiated (in a knownmanner) by the network operator and as such does not fall within thescope of the present document.

It is thus evident that, the basic principles of the invention remainingthe same, the details and embodiments may widely vary with respect towhat has been described and illustrated purely by way of example,without departing from the scope of the presented invention as definedin the annexed claims.

1-16. (canceled)
 17. A method of providing extra traffic paths in acommunication network comprising at least two protection channelsassociated to respective transmission channels, each of said at leasttwo protection channels admitting an active state for carrying, in thepresence of a failure in said associated transmission channel, trafficto be carried by the associated transmission channel and a stand-bystate, wherein the protection channel is adapted to carry extra traffic,comprising a step of running said at least two protection channels in asub-network connection protection scheme, whereby one of said at leasttwo protection channels in said stand-by state is adapted to ensurerecovery of extra traffic carried by the other of said at least twoprotection channels while one of the following conditions is met: saidother of said at least two protection channels is switched to saidactive state, and said other of said at least two protection channels issubject to failure.
 18. The method of claim 17, comprising the steps of:associating to each of said at least two protection channelscorresponding input and output digital cross connects; and running saidsub-network connection protection scheme at said input and outputdigital cross connects.
 19. The method of claim 17, comprising the stepsof: associating to each of said at least two protection channelscorresponding input and output add-drop multiplexers; and running saidsub-network connection protection scheme at said input and outputadd-drop multiplexers.
 20. The method of claim 17, comprising the stepof providing in said communication network at least one ring structureincluding non-coextensive paths and the step of associating said atleast two protecting channels to respective non-coextensive paths insaid ring.
 21. The method of claim 17, comprising the step of providingin said communication network a plurality of ring structures and thestep of associating said at least two protection channels to tworespective different rings of said plurality of rings.
 22. The method ofclaim 21, comprising the step of selecting said two different rings asrings belonging to the same class of rings.
 23. The method of claim 21,comprising the step of selecting said two different rings as ringsbelonging to different classes of rings.
 24. The method of claim 21,comprising the step of providing non-preemptible unprotected trafficcarried on non-preemptible channels in said network as well asnon-preemptible channels protected by a sub-network connectionprotection scheme, wherein said extra traffic is ensured an intermediatelevel of availability between the levels of protection provided by saidnon-preemptible channels and by said non-preemptible channels protectedby a sub-network connection protection scheme.
 25. A communicationnetwork comprising at least two protection channels associated torespective transmission channels, each of said at least two protectionchannels admitting an active state for carrying, in the presence of afailure in said associated transmission channel, traffic to be carriedby the associated transmission channel and a stand-by state, wherein theprotection channel is adapted to carry extra traffic, said at least twoprotection channels jointly defining a sub-network connection protectionscheme, whereby one of said at least two protection channels in saidstand-by state is adapted to ensure recovery of extra traffic carried bythe other of said at least two protection channels while one of thefollowing conditions is met: said other of said at least two protectionchannels is switched to said active state, and said other of said atleast two protection channels is subject to failure.
 26. The network ofclaim 25, comprising corresponding input and output digital crossconnects associated to each of said at least two protection channels andwherein said input and output digital cross connects jointly define saidsub-network connection protection scheme.
 27. The network of claim 25,comprising corresponding input and output add-drop multiplexersassociated to each of said at least two protection channels and whereinsaid input and output add-drop multiplexers jointly define saidsub-network connection protection scheme.
 28. The network of claim 25,comprising at least one ring structure including non-coextensive pathsand wherein said at least two protecting channels are associated torespective non-coextensive paths in said ring.
 29. The network of claim25, comprising a plurality of ring structures and wherein said at leasttwo protection channels are associated to two respective different ringsof said plurality of rings.
 30. The network of claim 29, wherein saidtwo different rings belong to the same class.
 31. The network of claim29, wherein said two different rings belong to different ring classes.32. A computer program product capable of being loaded in the memory ofat least one computer and including software code portions forperforming the steps of the method of any one of claims 17 to 24.